This invention relates to slide projectors and, more particularly, to an automatic focusing system for such a projector which is insensitive to the type of slide being projected for maintaining proper projection focus.
In the past, various types of automatic focusing systems for slide projectors have been proposed and implemented. In a relatively conventional automatic focusing system, the automatic focusing elements and the projection lens are mounted on a carriage which is movable with respect to the base of the projector. This carriage is between the projection station which holds a slide being projected and the surface upon which the image of the slide is projected for viewing. The projection lens is movable by an operator relative to the carriage to bring the image of the projected slide into focus on the viewing surface. The entire carriage, including the projection lens, is movable under control of the automatic focusing system to maintain constant the distance between the film within the slide mount at the projection station and the carriage, and hence between the slide film and the projection lens. Accordingly, although the film may "pop" with respect to the slide mount at the projection station after being exposed to heat from the projection lamp, focus is maintained. This has been accomplished in the past by reflecting light from the slide film onto a dual photocell arrangement. The outputs of the photocells are compared and the resultant comparison is utilized for energizing a reversible motor which moves the carriage relative to the slide film in order to balance the amount of light received by the two photocells.
While the aforedescribed arrangement may be generally satisfactory, under certain conditions of operation such an arrangement proves to be inadequate. One of the conditions under which the aforedescribed prior art arrangement becomes inadequate is when the slides being projected are randomly intermixed according to type. There are essentially two types of slides, an open, or bare, film slide and a glass covered film slide. If a system of the aforedescribed type is focused utilizing an open film slide, when a glass covered slide is at the projection station additional reflecting surfaces due to the presence of the glass change both the amount and distribution of the reflected light on the photocells. Accordingly, when the system moves the carriage to balance this light distribution, the projected image becomes defocused. It is therefore a specific object of the present invention to provide an automatic focusing system which allows the use of intermixed open film slides and glass covered film slides.